The use of a sequence of reference calibration patches exposed on a roll of film to enable better exposure control during optical printing is known in the art; see for example U.S. Pat. No. 5,767,983 issued Jun. 16, 1998 to Terashita entitled Color Copying Apparatus for Determining Exposure Amount from Image Data of an Original Image and a Reference Image. The use of reference calibration patches has also been shown to be useful in determining correction values for scanned film data used in digital printing. See for example U.S. Pat. No. 5,667,944 issued Sep. 16, 1997 to Reem et al. entitled Digital Process Sensitivity Correction, and U.S. Pat. No. 5,649,260 issued Jul. 15, 1997 to Wheeler et al. entitled Automated Photofinishing Apparatus.
Problems experienced with these reference calibration patches have included the occurrence of naturally arising artifacts in exposing the reference calibration patches onto the film, difficulties experienced in subsequent measurement of the patches on processed film, and data loss due to defects such as scratches or streaks in the images of the printed patches. To limit the area of film that is occupied by the reference calibration patches, it is desirable to expose as many reference calibration patches as possible onto the smallest area of film. In the exposure process, flare from high exposure reference calibration patches can affect the exposure of nearby patches, thereby affecting the intended exposure of the nearby patches. In the measurement process, flare can bias the accurate reading of the patch density, particularly in high density patches.
There is a need therefore for an improved reference calibration patch arrangement that minimizes the problems noted above.